Method of producing silver nanoparticles

ABSTRACT

A method of producing silver nanoparticles includes reducing, with a silver ion reducing agent, silver ions of 40 mM or more in a reaction solution in the presence of a particle protective agent and an element more noble than silver.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2019-088310 filed on May 8, 2019, incorporated herein by reference inits entirety.

BACKGROUND 1. Technical Field

The disclosure relates to a method of producing silver nanoparticles.

2. Description of Related Art

Metal nanoparticles, which may have properties different from those ofbulk materials, are used and studied to be used in various applicationssuch as, for example, catalysts, ink materials, and electroniccomponents.

Among the metal nanoparticles, silver nanoparticles have variousexcellent physical and chemical properties in terms of function, andvarious research and development have been made on uses and producingmethods thereof.

For example, Japanese Unexamined Patent Application Publication No.2010-285695 (JP 2010-285695 A) discloses silver fine particles obtainedby adding, to a polyol solvent, a silver compound having an averageparticle diameter of 10 μm or less in an amount of 1% by weight to 15%by weight, and a water-soluble polymer, having excellent adsorptivity tosilver and serving as a dispersant, in an amount of 5% by weight to 80%by weight with respect to a silver content of the silver compound, andthen reducing the solution by heating at 100° C. or lower. The silverfine particles have, each on its surface, a coating layer of thewater-soluble polymer, have an average particle diameter of 30 nm orless and a standard deviation a/average particle diameter d of 30% orless, and have monodispersity.

Japanese Unexamined Patent Application Publication No. 2011-225974 (JP2011-225974 A) discloses a method of producing silver nanoparticles, inwhich silver nitrate is reduced with citrate in water in the presence ofa hydrophilic polymer such as polyvinylpyrrolidone and/or polyvinylalcohol and an amine compound.

SUMMARY

In the field of electronic packaging, the metal nanoparticles haverecently been studied as a lead-free bonding material that can be bondedat low temperatures. It is difficult to bond a lead-free solder at 250°C. or lower. However, it is possible to bond a lead-free soldercontaining the metal nanoparticles at 250° C. or lower. This is achievedby utilizing the property of the metal nanoparticles having a lowermelting point compared to bulk materials, whereas having a melting pointequivalent to that of bulk materials after the metal nanoparticles areused for bonding and sintered.

To use the metal nanoparticles, particularly the silver nanoparticles,as a high heat-resistant bonding material, it is necessary to keep themelting point of the silver nanoparticles constant. In order to keep themelting point of the silver nanoparticles constant, it is desirable tonarrow a particle size distribution of the silver nanoparticles toimprove dispersibility.

However, the method described in JP 2010-285695 A requires, for example,a step of pulverizing a silver compound to prepare a silver compoundhaving a small particle diameter, and the method is complicated.

Further, it is presumed that the dispersibility of the silvernanoparticles obtained by the method described in JP 2011-225974 A isnot uniform. The method described in JP 2011-225974 A includes areaction system of silver nitrate and sodium citrate using water as asolvent. In the reaction system, a nucleation rate of silver variesparticularly at low temperatures. This widens the particle sizedistribution of the silver nanoparticles, and as a result, thedispersibility of the obtained silver nanoparticles is not uniform.

Specifically, in a reaction system in which only the same element as thematerial of the nanoparticles, namely, silver, is present, after silvernuclei are formed, silver preferentially precipitates and grows on thesilver nuclei. Meanwhile, silver nuclei are formed on a free surfacelocally having high energy. Since particle growth and nucleation occursimultaneously in different reaction fields, the particle diameter ofthe silver nanoparticles is not uniform.

When a tracing experiment according to JP 2011-225974 A was actuallyperformed, the silver nanoparticles obtained by the method described inJP 2011-225974 A had a standard deviation a/average particle diameter d,which is an index of the dispersibility described in JP 2010-285695 A,of 43%, and did not fall below 30%.

The disclosure provides a method of producing silver nanoparticleshaving a narrow particle size distribution.

The inventors of the disclosure have studied various means for solvingthe above-described problems, and have found that, in a method ofproducing silver nanoparticles, in which silver ions in a reactionsolution are reduced with a silver ion reducing agent in the presence ofa particle protective agent, it possible to produce silver nanoparticleshaving a narrow particle size distribution by adjusting theconcentration of silver ions to be constant and adding an element morenoble than silver, and have thus completed the disclosure.

An aspect of the disclosure relates to a method of producing silvernanoparticles. The method includes reducing, with a silver ion reducingagent, silver ions of 40 mM or more in a reaction solution in thepresence of a particle protective agent and an element more noble thansilver. In the above aspect, the element more noble than silver may bepalladium. In the above aspect, an amount of palladium may be 0.05% byweight to 20% by weight as a palladium metal, based on a weight ofsilver as a metal. In the above aspect, a solvent used for the reactionsolution may be water. In the above aspect, the silver ion reducingagent may be citrate. In the above aspect, the particle protective agentmay be polyvinylpyrrolidone. In the above aspect, the method may becarried out using a microwave synthesizer.

According to the disclosure, silver nanoparticles having a narrowparticle size distribution can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like signs denote likeelements, and wherein:

FIG. 1 is a graph showing a result of a particle size distribution ofsilver nanoparticles prepared in a first embodiment;

FIG. 2 is a graph showing a result of a particle size distribution ofsilver nanoparticles prepared in Comparative Example 1; and

FIG. 3 is a graph showing a relationship between silver ionconcentration and a particle size variation index in the firstembodiment, a second embodiment, and Comparative Examples 1 to 4.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, preferable embodiments of the disclosure will be describedin detail. In this specification, features of the disclosure will bedescribed with reference to FIGS. 1 to 3 as appropriate. A method ofproducing silver nanoparticles according to the disclosure is notlimited to the embodiments described below, but may be carried out invarious forms in which modifications and improvements may be made bythose skilled in the art without departing from the scope of thedisclosure.

The disclosure relates to the method of producing the silvernanoparticles, which includes reducing, with a silver ion reducingagent, silver ions having constant concentration in a reaction solutionin the presence of a particle protective agent and an element more noblethan silver.

Here, a raw material of the silver ions is not limited, but may include,for example, an inorganic salt such as hydrochloride, sulfate, nitrateand phosphate of silver, and an organic salt such as carboxylate andsulfonate of silver. In the disclosure, silver nitrate, which isinexpensive, is preferably used as the raw material for the silver ions.

The concentration of the silver ions in the reaction solution is 40mmol/L (mM) or more, preferably 50 mM or more. An upper limit of theconcentration of the silver ions in the reaction solution is not limitedas long as the raw material of the silver ions is present as silver ionsin the reaction solution, but is normally 500 mM, preferably 400 mM.

By setting the concentration of the silver ions in the reaction solutionin the above range, variations in the obtained silver nanoparticles arereduced, in other words, a particle size distribution of the obtainedsilver nanoparticles is narrowed.

A solvent used for the reaction solution in the method according to thedisclosure is not limited. Examples of the solvent used in the reactionsolution in the method according to the disclosure include a low-boilingsolvent having a boiling point of 300° C. or lower. Examples of thelow-boiling solvent include, but are not limited to, low-boiling polarsolvents such as water, alcohol such as ethanol, other organic solvents,and a mixture of two or more thereof. Water is preferably used as thesolvent used for the reaction solution.

In the disclosure, by using a low-boiling solvent as the solvent usedfor the reaction solution, the handleability of the solvent can beimproved and a burden on the environment can be reduced.

The particle protective agent is a compound that binds to a part or theentire surface of the silver nanoparticles suspended in the solvent, andis a compound that suppresses aggregation of the silver nanoparticles.Examples of the particle protective agent include, but are not limitedto, polyvinylpyrrolidone (PVP), thiol-based agents, and polyvinylalcohol (PVA). PVP is preferably used as the particle protective agent.

An amount of the particle protective agent is not limited and can bechanged depending on a desired particle diameter of the silvernanoparticles.

By using the particle protective agent in the disclosure, theaggregation of the generated silver nanoparticles can be suppressed.

An element more noble than silver is an element that is at a higherpotential than silver in standard potential series, in other words, anelement that has a lower ionization tendency than silver. Examples ofthe element more noble than silver include, but are not limited to,palladium (Pd), iridium (Ir), platinum (Pt), and gold (Au). Palladium ispreferably used as the element more noble than silver.

The element more noble than silver may be in the form of cations or inthe form of nanoparticles. When the element more noble than silver is inthe form of cations, the element more noble than silver may be presentas an inorganic salt such as hydrochloride, sulfate, nitrate andphosphate, or an organic salt such as carboxylate and sulfonate.

An amount of the element more noble than silver is, as an element,normally 0.01% by weight to 20% by weight, preferably 0.05% by weight to20% by weight, based on the weight of silver as a metal.

In particular, when the element more noble than silver is palladium, theamount of palladium is, as a palladium metal, normally 0.05% by weightto 20% by weight based on the weight of silver as a metal.

Alternatively, a weight ratio of palladium to silver as a metal is inthe range where silver is normally 5 to 20 when palladium is regarded as1 (1:5 to 1:20).

By adding the element more noble than silver to the reaction system, theelement more noble than silver forms nuclei of the nanoparticles beforethe silver ions do, so that the silver ions can precipitate and grow assilver on the nuclei formed by the element more noble than silver. As aresult, the silver nanoparticles having a narrow particle sizedistribution can be produced even when the silver nanoparticles areproduced at low temperatures.

A silver ion reducing agent is a material that can reduce the silverions to silver having an oxidation number of 0 through anoxidation-reduction reaction.

Examples of the silver ion reducing agent include, but are not limitedto, citric acid or a salt thereof, such as trisodium citrate, disodiumcitrate, monosodium citrate, oxalic acid or a salt thereof, such assodium oxalate, and ascorbic acid or a salt thereof, such as sodiumascorbate. Citrate is preferably used as the silver ion reducing agent.

The silver ion reducing agent can also act as a reducing agent for theelement more noble than silver when the element more noble than silveris present in the form of cations.

The amount of the silver ion reducing agent is not limited as long asthe silver ions and, in some cases, the element more noble than silvercan be reduced to a metal having the oxidation number of 0 through theoxidation-reduction reaction.

In the disclosure, besides the above materials,ethylenediaminetetraacetic acid (EDTA) and/or a salt thereof can beadded.

In the disclosure, the order of adding each material, a temperatureduring the addition, a mixing method, a mixing time, and the like arenot limited, and the materials are mixed to prepare a uniform reactionsolution. In the disclosure, the reaction is started after the uniformreaction solution is prepared.

The disclosure can be carried out by a conventional heating method or amethod using a microwave synthesizer.

In the disclosure, in the conventional heating method, a reactiontemperature is not limited, but is normally 300° C. or lower.

In the disclosure, in the conventional heating method, the reaction timeis not limited, but is normally 1 hour to 100 hours, for example, 1 hourto 3 hours, and preferably 24 hours to 100 hours.

In the disclosure, in a method using the microwave synthesizer, thereaction solution is irradiated with microwaves to promote the reaction.Thus, a polar solvent is used as a solvent contained in the reactionsolution. The polar solvent absorbs the microwaves when irradiated withthe microwaves and converts the microwaves into heat energy, therebygenerating heat. Examples of the polar solvent include, but are notlimited to, a low-boiling polar solvent such as water and ethanol. Wateris preferably used as the polar solvent.

In the method using the microwave synthesizer, a material of a containerfor accommodating the reaction solution is not limited as long as a rawmaterial solution can be uniformly irradiated with the microwaves. Forexample, when irradiating the raw material solution with the microwavesfrom outside a reactor through the reactor, a material that transmitsmicrowaves, for example, ceramics and glass, can be used. When directlyirradiating the raw material solution with the microwaves from aposition above the raw material solution, a material that reflects themicrowaves, for example, a metal such as aluminum and stainless steelcan be used.

In the method using the microwave synthesizer, the microwaves aregenerated by a microwave irradiation source (microwave oscillator(magnetron)), and the microwave irradiation source can use either asingle mode system or a multi-mode system.

In the method using the microwave synthesizer, an output of themicrowave irradiation source is not limited and can be appropriatelychanged depending on reaction conditions, for example, a type of thereaction, but based on a total volume of the reaction solution, isnormally 100 W/L to 10 kW/L, preferably 100 W/L to 5 kW/L.

In the method using the microwave synthesizer, a frequency of themicrowaves generated by the microwave irradiation source is not limitedand can be appropriately changed, but is normally 1 GHz to 10 GHz,preferably 2 GHz to 6 GHz. In the disclosure, the frequency of anindustrial microwave power supply of 2.45 GHz is preferably used as themicrowave frequency.

In the method using the microwave synthesizer, a temperature of thereaction solution heated by the irradiation of the microwaves is notlimited and can be appropriately changed depending on the reactionconditions. The temperature of the reaction solution heated by theirradiation of the microwaves only needs to be equal to or lower thanthe boiling point of the solvent.

In the method using the microwave synthesizer, an irradiation time ofthe microwaves to the reaction solution is not limited and can beappropriately changed depending on the reaction conditions, but isnormally 1 minute to 200 minutes, preferably 1 minute to 80 minutes.Alternatively, the reaction solution can be irradiated with themicrowaves so as to maintain a target temperature of the reactionsolution.

In the method using the microwave synthesizer, a total reaction timeincluding the irradiation time of the microwaves is not limited and canbe appropriately changed depending on the reaction conditions, but is,for example, 1 minute to 300 minutes, preferably 1 minute to 80 minutes.

The disclosure is preferably carried out using the microwave synthesizerfrom the viewpoint that the entire reaction field can be uniformlyheated.

In the disclosure, the reaction solution is preferably stirred by astirring mechanism such as a propeller stirrer and a vibration stirrer.By stirring the reaction solution, the silver nanoparticles generated inthe reaction solution can be uniformly dispersed, and the reactionsolution can be kept uniform.

The disclosure may be carried out through a batch system or a flowsystem. The disclosure is preferably carried out through the batchsystem. By carrying out the disclosure through the batch system, asynthesis reaction itself can be completed, and a yield of the obtainedsilver nanoparticles can be improved. Further, the concentration of thereaction solution can be made high, and clogging of a piping of thesilver nanoparticles which may occur in the flow system is restrained.

The solution containing the silver nanoparticles obtained according tothe disclosure may be subjected to processes such as separation andpurification (for example, salting out or centrifugation) by a methodknown in the technical art to obtain target silver nanoparticles and/ora dispersion containing the target silver nanoparticles.

The silver nanoparticles produced by the method according to thedisclosure have uniform particle diameters, that is, a narrow particlesize distribution.

The silver nanoparticles produced by the method according to thedisclosure can be used as a high heat-resistant lead-free bondingmaterial in the field of electronic packaging, in addition toconventional catalysts, electronic components, ink materials, and thelike.

Hereinafter, several embodiments related to the disclosure will bedescribed. The disclosure is not intended to be limited to theembodiments.

1. Preparation of Silver Nanoparticles First Embodiment

Each of silver nitrate, EDTA, trisodium citrate, and palladium nitratewere dissolved in water. First, an aqueous solution of EDTA was added toan aqueous solution of silver nitrate and the mixture was stirred.Second, an aqueous solution of sodium citrate was added and the mixturewas stirred until a silver-EDTA precipitate dissolved. Third, an aqueoussolution of palladium nitrate was added and the mixture was stirred.Fourth, an aqueous solution of PVP-polyacrylic acid (PAA) whose pH hadbeen neutralized with sodium hydroxide added thereto was added and themixture was stirred. Finally, purified water was added and the mixturewas stirred to obtain the reaction solution of the concentration shownin Table 1. The concentration of palladium in the reaction solution was1.3% by weight as a palladium metal based on the weight of silver as ametal.

TABLE 1 Silver nitrate 300 mM EDTA 300 mM Sodium citrate 900 mM PVP 50mM Palladium nitrate 4 mM

The reaction solution was heated at 90° C. for 80 minutes in themicrowave synthesizer to obtain the silver nanoparticles.

Second Embodiment

The reaction was carried out in the same manner as in the firstembodiment except that the concentration of each material was adjustedto the concentration shown in Table 2 to obtain a reaction solution,thereby obtaining the silver nanoparticles.

TABLE 2 Silver nitrate 50 mM EDTA 50 mM Sodium citrate 150 mM PVP 8.3 mMPalladium nitrate 0.67 mM

Comparative Example 1

The reaction was carried out in the same manner as in the firstembodiment except that palladium nitrate was not added, to obtain thesilver nanoparticles. Table 3 shows the concentration of each materialin the reaction solution of Comparative Example 1.

TABLE 3 Silver nitrate 300 mM EDTA 300 mM Sodium citrate 900 mM PVP 50mM

Comparative Example 2

The reaction was carried out in the same manner as in ComparativeExample 1 except that the concentration of each material was adjusted tothe concentration shown in Table 4 to obtain the reaction solution,thereby obtaining the silver nanoparticles.

TABLE 4 Silver nitrate 50 mM EDTA 50 mM Sodium citrate 150 mM PVP 8.3 mM

Comparative Example 3

The reaction was carried out in the same manner as in ComparativeExample 1 except that the concentration of each material was adjusted tothe concentration shown in Table 5 to obtain the reaction solution,thereby obtaining the silver nanoparticles.

TABLE 5 Silver nitrate 3.3 mM EDTA 3.3 mM Sodium citrate 9.9 mM PVP 0.55mM

Comparative Example 4

The reaction was carried out in the same manner as in ComparativeExample 1 except that the concentration of each material was adjusted tothe concentration shown in Table 6 to obtain the reaction solution,thereby obtaining the silver nanoparticles.

TABLE 6 Silver nitrate 5 mM EDTA 5 mM Sodium citrate 15 mM PVP 0.83 mM

2. Evaluation of Silver Nanoparticles

The silver nanoparticles obtained in the first embodiment andComparative Example 1 were each measured by a particle size distributionanalyzer (dynamic light scattering (DLS) method).

FIG. 1 shows the results of the particle size distribution of the silvernanoparticles prepared in the first embodiment, and FIG. 2 shows theresults of the particle size distribution of the silver nanoparticlesprepared in Comparative Example 1.

As shown in FIGS. 1 and 2, the silver nanoparticles of the firstembodiment in which palladium, which is an element more noble thansilver, was added to the reaction solution, had a standard deviationa/average particle diameter d, which is an index of dispersibility, of24%, that is, 30% or less. On the other hand, the silver nanoparticlesof Comparative Example 1 in which palladium, which is an element morenoble than silver, was not present in the reaction solution, had astandard deviation a/average particle diameter d, which is an index ofdispersibility, of 43%.

The reasons of the above results can be considered as follows. By addingpalladium, which is more noble than silver, to the reaction system,palladium, which is more noble than silver, formed nuclei of thenanoparticles before the silver ions did. The silver ions precipitatedand grew as silver on the nuclei formed by palladium, which is morenoble than silver. As a result, silver nanoparticles having a narrowerparticle size distribution could be produced.

3. Examination Experiment of Silver Ion Concentration

A dispersion liquid in which the silver nanoparticles are dispersed(silver nanoparticle dispersion liquid) shows an absorption spectrumthat is dependent on an average particle diameter. For example, when twosilver nanoparticle dispersion liquids include different averageparticle diameters, the absorption spectra of the silver nanoparticledispersion liquids may also be different.

Considering such properties of the silver nanoparticle dispersionliquid, when silver nanoparticles having different particle diametersare present in a silver nanoparticle dispersion liquid, the absorptionspectrum of the silver nanoparticle dispersion liquid correspond to thesum of the absorption spectra of the silver nanoparticles havingdifferent particle diameters, and a peak width of an absorption peak inthe absorption spectrum of the silver nanoparticle dispersion liquid maybecome large. In other words, as the variations in the particle size ofthe silver nanoparticles in the silver nanoparticle dispersion liquidincrease, the peak width of the absorption peak in the absorptionspectrum of the silver nanoparticle dispersion liquid may also increase.

Utilizing such properties of the silver nanoparticle dispersion liquid,the absorption spectra of the second embodiment and Comparative Examples1 to 4 were measured, a difference between two absorption wavelengths athalf the intensity of the maximum absorption peak wavelength and themaximum absorption peak wavelength were calculated from each of theabsorption spectra, and a quotient of the two values was obtained as a“particle size variation index”.

Particle size variation index=Difference between two absorptionwavelengths at half the intensity of the maximum absorption peakwavelength/Maximum absorption peak wavelength

FIG. 3 shows the relationship between the silver ion concentration andthe particle size variation index. As the particle size variation indexof the first embodiment, 0.24 was used as the standard deviationσ/average particle diameter d, which is an index of dispersibility.Considering the fact that the smaller the particle size variation indexis, the smaller the variations in silver nanoparticle size is, FIG. 3shows that when the silver ion concentration is around 40 mM, forexample, 50 mM, the obtained silver nanoparticles showed a smallvariation and the variations in the silver nanoparticles was kept smalleven when the silver ion concentration was set higher.

What is claimed is:
 1. A method of producing silver nanoparticles, themethod comprising reducing, with a silver ion reducing agent, silverions of 40 mM or more in a reaction solution in a presence of a particleprotective agent and an element more noble than silver.
 2. The methodaccording to claim 1, wherein the element more noble than silver ispalladium.
 3. The method according to claim 2, wherein an amount ofpalladium is 0.05% by weight to 20% by weight as a palladium metal,based on a weight of silver as a metal.
 4. The method according to claim1, wherein a solvent used for the reaction solution is water.
 5. Themethod according to claim 1, wherein the silver ion reducing agent iscitrate.
 6. The method according to claim 1, wherein the particleprotective agent is polyvinylpyrrolidone.
 7. The method according toclaim 1, wherein the method is carried out using a microwavesynthesizer.